The present invention finds application in the field of radio frequency power amplifiers, and more particularly to power amplifiers used in base stations under digital spread spectrum modulation and in a pHEMT device for power amplifier operation over wide temperature range.
One example of spread-spectrum technology includes Wideband Code Division Multiple Access (WCDMA). WCDMA spreads multiple conversations across a wide segment of the spectrum as opposed to splitting a channel into time slots. With WCDMA, unique digital codes are used to differentiate subscribers that are simultaneously using the same spectrum.
In order to isolate conversations between two adjacent channels, a power amplifier used for WCDMA requires a certain level of linearity, characterized by an output power and an associated adjacent-channel power ratio (ACPR). As an example, an output power on the order of 30 dBm with an associated ACPR on the order of −45 dBc (decibels with respect to the carrier) is specified as the power and linearity requirement for the active device in a power amplifier. Linear RF power amplifier manufacturers can utilize such a device having an ACPR at or below −45 dBc. The exact degree of power and linearity depends on the application and can vary widely.
In addition, base stations that utilize such a power amplifier are installed across the world and must be capable of operating over a wide range of temperatures to provide a desired level of service coverage. Accordingly, the RF power amplifier circuit should maintain performance over a temperature range in the order of −40° C. to +90° C. at its mounting base plate. Only minimal performance degradation is allowed at the extreme temperature specifications.
In radio frequency base station power amplifier operations, silicon LDMOS technology typically is used when the frequency is below 2.5 GHz. However, silicon (Si) LDMOS has insufficient efficiency to meet competitive requirements above 2.5 GHz. Therefore, semiconductor technologies, such as GaAs (or AlGaAs) and GaN have been proposed and increasingly adopted. Typically, these are Metal Semiconductor Field Effect Transistors (MESFET) or High Electron Mobility Transistors (HEMT) including pseudomorphic High Electron Mobility Transistors (pHEMT).
When an AlGaAs pHEMT device is used for a power amplifier under digital spread spectrum modulation stimulus at −40° C., ACPR can degrade dramatically, as compared to room temperature. This level of linearity degradation phenomenon occurs under digital modulation, such as W-CDMA on the order of from 2 dB to 10 dB. It would be desired to obtain a device that does not degrade at this temperature range. Further, it would be desired to maintain performance of the pHEMT device at higher temperatures.
Accordingly, it would be desirable to provide for reduced power degradation under digital modulation over wide temperature operation for overcoming the problems in the pHEMT art. It would further be advantageous if the new pHEMT was compatible with existing manufacturing processes and materials used in semiconductor manufacturing. The present invention addresses one or more of these needs.